1. Field of the Invention
The present invention relates to a fuel injection control apparatus for an internal combustion engine. More particularly, it relates to such fuel injection control apparatus which detects a state of the internal combustion engine being warmed from a temperature of cooling water, and controls fuel injection starting timing on the basis of a detection signal.
2. Discussion of Background
FIGS. 1 and 2 are diagrams showing a conventional fuel injection control apparatus for an internal combustion engine described in, for instance, Japanese Unexamined Patent Publication No. 173826/1988. In FIGS. 1 and 2, the internal combustion engine is to be controlled by an electronic control circuit such as a microcomputer. A throttle valve 8 is disposed at the downstream side of an air cleaner (not shown), and a surge tank 12 is provided at the downstream side of the throttle valve 8. A pressure sensor 6 is attached to the surge tank 12 to detect a pressure in an intake air pipe. A bypass passage 14 is formed to communicate the upstream side of the throttle valve 8 with the surge tank 12 formed at the downstream side of the throttle valve 8 so as to bypass the valve 8. An idle speed control valve (ISC) 16B is attached to the bypass passage 14. The degree of opening of the ISC valve 16B is adjusted by a pulse motor 16A. The surge tank 12 is communicated with the combustion chamber of the engine 20 through an intake manifold 18, an air intake port 22 and an air intake valve 23. An electromagnetic type fuel injector 24 is attached to each cylinder so as to project in the intake manifold 18.
The combustion chamber of the engine 20 is communicated with a catalyst device (not shown) filled with a three-component catalyst through a discharge valve 25, a discharge port 26 and an exhaust manifold 28. The exhaust manifold 28 includes an O.sub.2 sensor 30. A cooling water temperature sensor 34 is attached to an engine block 32 so as to extend into a water jacket through the engine block 32. The cooling water temperature sensor 34 detects the temperature of cooling water for the engine and outputs a signal indicative of the water temperature which represents the temperature of the engine. The temperature of oil for the engine may also be detected to detect the temperature of the engine.
An ignition plug 38 is provided for each cylinder so that it penetrates the cylinder head 36 of the engine 20 so as to extend in the combustion chamber. Each of the ignition plugs 38 is connected to an electronic control unit (ECU) 44 constituted by a microcomputer or the like through a distributor 40 and an igniter 42. A cylinder discriminating sensor 46 and a revolution angle sensor 48 are attached to the distributor 40.
As shown in FIG. 2, the electronic control circuit 44 comprises a microprocessor 60, a read-only-memory (ROM) 62, a random-access-memory (RAM) 64, a back-up RAM (BU-RAM) 66, an I/O port 68, an input port 70, output ports 72, 74 and buses such as data buses, control buses and so on which connect the above-mentioned elements.
The I/O port 68 is connected with an analog/digital (A/D) transducer 78 and a multiplexer 80. The multiplexer 80 is, on one hand, connected with the pressure sensor 6 through a filter 7 and a buffer 82, and is, on the other hand, connected with the cooling water temperature sensor 34 through a buffer 84.
The microprocessor (MPU) 60 is so adapted as to control the multiplexer 80 and the A/D transducer 78 whereby the output of the pressure sensor 6 which is provided through the filter 7 and the output of the cooling water temperature sensor 34 are sequentially transduced into digital signals which are to be stored in the RAM 64. Thus, the multiplexer 80, the A/D transducer 78 and the MPU 60 and so on function as a sampling means which samples the outputs of the pressure and temperature sensors at predetermined time intervals.
The input port 70 is, on one hand, connected with a comparator 88, a buffer 86 and the O.sub.2 sensor 30, and is, on the other hand, connected with the cylinder discriminating sensor 46 and the revolution angle sensor 48 through a waveform shaping circuit 90.
The output port 72 is connected with the igniter 42 through a driving circuit 92.
The output port 72 is connected with the fuel injection valve 24 through a driving circuit 94 provided with a downcounter. A numeral 98 designates a clock and a numeral 99 designates a timer. The ROM 62 stores programs such as a control routine which is described below.
The control routine stored in the ROM 62 will be described.
FIG. 8 shows a main routine executed at each predetermined time (for instance 4 msec).
At Step 100, an engine revolution speed NE and an intake air pipe pressure PM are read, and a basic fuel injection time TP is calculated on the basis of the engine revolution speed NE and the intake air pipe pressure PM at Step 102. Then, a fuel injection time TAU is calculated by correcting the basic fuel injection time TP by using the temperature of air sucked into the engine, the temperature of cooling water for the engine or another suitable factor at Step 104.
FIG. 7 shows an interruption routine interrupted at each crank angle which is determined on the basis of the intake air pipe pressure and the engine revolution speed as shown in FIG. 9. When the interruption routine is started, the time of starting fuel injection .theta..sub.i as the latest information is calculated on the basis of the engine revolution speed NE and the intake air pipe pressure PM from a map representing the time of starting fuel injection as in FIG. 10, at Step 110.
Then, comparison of the absolute value .vertline..theta..sub.i -.theta..sub.i-1 .vertline. between the time of starting fuel injection .theta..sub.i as the latest information and the time of starting fuel injection .theta..sub.i-1 which has been obtained by calculation at the last time with a predetermined value K.sub.1, in order to eliminate hunting at the time of fuel injection, at Step 112. When the absolute value .vertline..theta..sub.i -.theta..sub.i-1 .vertline. is greater than the predetermined value K.sub.1, it is understood that there is a large amount of error between a desired value for the time of starting fuel injection, which is determined depending on an engine revolution speed and a load in the engine at an approximate time when the intake air valve is closed, and the operated value concerning the time of starting fuel injection, namely, it is judged that a misfire may occur. Then, a fuel injection time correction value .DELTA..theta. is operated in accordance with the following equation at Step 116: EQU .DELTA..theta.=(.theta..sub.i -.theta..sub.i-l).multidot.K.sub.2 ( 1)
where K.sub.2 is a correction coefficient which is represented by .beta./.alpha., i.e. the ratio of a crank angle .alpha. between the operation timing C2 at which the time of starting fuel injection as the latest information has been obtained by calculation and the operation timing C1 prior to the operation timing C2, to the crank angle .beta. between the operation timing C2 and the time of starting fuel injection.
When the absolute value .vertline..theta..sub.i -.theta..sub.i-1 .vertline. is lower than the predetermined value K.sub.1, it is understood that there is a small amount of error between the desired value for the time of starting fuel injection and an operated value concerning the time of starting fuel injection, i.e. it is judged that a misfire may not occur, and then, the correction value .DELTA..theta. is changed to 0 at Step 114.
At Step 118, the time of starting fuel injection, which is to be executed, is calculated by adding the correction value .DELTA..theta. to the time of starting fuel injection .theta..sub.i as the latest information, which has been obtained by calculation.
At Step 120, the time of starting fuel injection .theta..sub.i as the latest information is changed to the time of starting fuel injection .theta..sub.i-1 which has been operated at the last time to thereby rewrite the time of starting fuel injection used at the last time.
FIG. 11 shows an interruption routine interrupted when the time of starting fuel injection .theta..sub.i to be executed, which has been operated at Step 118 in FIG. 7, is obtained. When the interruption routine is started, the electromagnetic coil for a fuel injection valve is actuated to thereby start fuel injection at Step 122. At Step 124, a fuel injection time TAU which has been operated in the main routine is read. At Step 126, the time of stopping fuel injection is operated by adding the fuel injection time TAU to the present time (the time of starting fuel injection), and then the time of stopping fuel injection is set as the time of stopping a current which is to be fed to the fuel injection valve, to a downcounter for the driving circuit 94. The downcounter continues counting-down until the time of stopping current.
FIG. 12 shows an interruption routine interrupted when a counted value by the downcounter becomes 0. When the count value by the downcounter is 0, current supply to the fuel injection valve is stopped at Step 128 to thereby finish the fuel injection.
In the internal combustion engine provided with the conventional fuel injection apparatus which operates the time of starting fuel injection on the basis of the engine revolution speed and the load of the engine in consideration of a transient time in acceleration or deceleration operations, when the engine is just started or is not sufficiently warmed, the temperature of cooling water for the engine, i.e. a state of warming-up of the engine is not considered in the determination of the time for fuel injection. Accordingly, there has been a problem that the diameter of fuel droplets in a form of mist sucked in the cylinders is relatively large due to the reduction in temperature of the cooling water, and exhaust gas contains a large amount of unburned hydrocarbon (HC). As shown in FIG. 5, a relation of the time of starting fuel injection and an amount of discharged hydrocarbon varies depending on a state of warming-up of the engine. In particular, the diameter of the fuel in a form of mist becomes small by the ejection of the fuel during an air intake stroke under the condition of the temperature of the cooling water being 0.degree. C. or lower so as to obtain fuel-atomizing effect by the intake air. Therefore, the fuel is well mixed with air, whereby a time A in which an amount of discharged hydrocarbon is reduced, is obtainable during a fuel injection time. Since the viscosity of the fuel increases and the surface tension assumes a greater figure under the condition of the temperature of 0.degree. C. or lower. Accordingly, when the fuel is ejected in a time other than the time period A during the intake air stroke, a film of fuel having a thickness of about 1 mm or more is formed on the intake air valve and the wall of the intake air port, and there may occur a phenomenon that the fuel is sucked in the cylinder while it keeps a state of film of fuel without being atomized at the corner portions of the intake air valve when it begins to open.
Thus, under the low temperature conditions which are difficult to expect the function of atomizing the fuel at or around the intake air valve, it is preferable that the fuel injection is started at a time period in which a flow rate of intake air is high and an amount of the raising the intake air valve is small, i.e. the time period A in FIG. 5. Then, the diameter of the fuel in a form of mist sucked in the cylinder is small to thereby provide good conditions for combustion, whereby an amount of hydrocarbon in exhaust gas is reduced.
On the other hand, after the engine has been sufficiently warmed, the film thickness of the fuel deposited is small and the fuel on the intake air valve and the intake air port is atomized at the corner portions of the intake air valve. Accordingly, if fuel injection is carried out in the period other than the air intake stroke to thereby form a fuel film on the wall portion, the diameter of fuel in a form of mist in the cylinder is small, hence, an amount of hydrocarbon discharged is small. Thus, by detecting the temperature of cooling water for the engine, and by starting fuel injection in the time period A in the air intake stroke if the temperature of the engine is lower than a predetermined value, an amount of hydrocarbon discharged from the engine is reduced.